display: when a message gets overwritten, note that it is cleared

When on a one-row terminal a message gets automatically "dropped"
after a few moments, the subsequent waiting for a keystroke should
not think that there is still a message on the status bar.

This fixes https://savannah.gnu.org/bugs/?60591.

Bug existed since commit 8d974cd2 from two days ago.

src/winio.c

@@ -217,6 +217,7 @@ void read_keys_from(WINDOW *win)
 
 			if (input == ERR) {
 				if (LINES == 1) {
+					lastmessage = VACUUM;
 					edit_refresh();
 					curs_set(1);
 				} else